Get the pattern from draadje's blog: Free Pattern: Minecraft dirt block cube
I used 4 ply cotton yarn and a 2.5 mm hook for mine. The finished size was about 5cm square.
I followed the instructions for bock A exactly, but on block B the colour change back to green wasn't exactly where I wanted it. I did the green/brown row at row 34 and the last two rows green.
Once sewn up I added a little clip so you can use the block as a keyring or clip it to your schoolbag.